Thursday, July 21, 2016

This
is one slice through the map of the large-scale structure of the
Universe from the Sloan Digital Sky Survey and its Baryon Oscillation
Spectroscopic Survey. Each dot in this picture indicates the position of
a galaxy 6 billion years into the past. The image covers about 1/20th
of the sky, a slice of the Universe 6 billion light-years wide, 4.5
billion light-years high, and 500 million light-years thick. Colour
indicates distance from Earth, ranging from yellow on the near side of
the slice to purple on the far side. Galaxies are highly clustered,
revealing superclusters and voids whose presence is seeded in the first
fraction of a second after the Big Bang. This image contains 48,741
galaxies, about 3% of the full survey dataset. Grey patches are small
regions without survey data. Image credit: Daniel Eisenstein and SDSS-III.Hi-res image

What are the properties of Dark Energy? This question is one of the most intriguing ones in astronomy and scientists are one step closer in answering this question with the largest three-dimensional map of the universe so far: This map contains 1.2 million galaxies in a volume spanning 650 cubic billion light years. Hundreds of scientists from the Sloan Digital Sky Survey III (SDSS-III) – including researchers at the Max Planck Institutes for Extraterrestrial Physics and for Astrophyics - used this map to make one of the most precise measurements yet of dark energy. They found excellent agreement with the standard cosmological model and confirmed that dark energy is highly consistent with a cosmological constant.

"We have spent a decade collecting measurements of 1.2 million
galaxies over one quarter of the sky to map out the structure of the
Universe over a volume of 650 cubic billion light years,” says Jeremy
Tinker of New York University, a co-leader of the scientific team that
led this effort. Hundreds of scientists are part of the Sloan Digital
Sky Survey III (SDSS-III) team.

These new measurements were carried out by the Baryon Oscillation
Spectroscopic Survey (BOSS) programme of SDSS-III. Shaped by a
continuous tug-of-war between dark matter and dark energy, the map
revealed by BOSS allows astronomers to measure the expansion rate of the
Universe by determining the size of the so-called baryonic acoustic
oscillations (BAO) in the three-dimensional distribution of galaxies.

Pressure waves travelled through the young Universe up to when it was
only 400,000 years old at which point they became frozen in the matter
distribution of the Universe. The end result is that galaxies are
preferentially separated by a characteristic distance, which astronomers
call the BAO scale. The primordial size of the BAO scale is exquisitely
determined from observations of the cosmic microwave background.

Ariel Sanchez of the Max-Planck Institute of Extraterrestrial Physics
(MPE) led the effort to estimate the exact amount of dark matter and
dark energy based on the BOSS data and explains: "Measuring the acoustic
scale across cosmic history gives a direct ruler with which to measure
the Universe’s expansion rate. With BOSS, we have traced the BAO’s
subtle imprint on the distribution of galaxies spanning a range of time
from 2 to 7 billion years ago."

For the very precise measurements, however, the data had to be
painstakingly analysed. Especially the determination of distances to the
galaxies posed a big challenge. This is inferred from the galaxy
spectra, which show that a galaxy’s light is shifted to the red part of
the spectrum because it moves away from us. This so-called redshift is
correlated with a galaxy’s distance: The farther a galaxy is away from
us, the faster it moves.

“However, galaxies also have peculiar motions and the peculiar
velocity component along the line-of-sight leads to the so-called
redshift space distortion,” explains Shun Saito from the Max Planck
Institute for Astrophysics (MPA), who contributed sophisticated models
to the BOSS data analysis. “This makes the galaxy distribution
anisotropic because the line-of-sight direction is now special – only
along this direction the distance is measured through a redshift, which
is contaminated by peculiar velocity. In other words, the characteristic
anisotropic pattern allows us to measure the peculiar velocity of
galaxies – and because the motion of galaxies is governed by gravity, we
can use this measurement to constrain to what level Einstein’s general
relativity is correct at cosmological scales. In order to properly
interpret the data, we have developed a refined model to describe the galaxy distribution.”

Another approach, used by a junior MPE researcher for his PhD thesis,
is to use the angular positions of galaxies on the sky instead of
physical 3D positions. “This method uses only observables,” explains
Salvador Salazar. “We make no prior assumptions about the cosmological
model.”

Around the world, other groups all used slightly different models and
methodologies to analyse the huge BOSS data set. “We now have seven
measurements, which are slightly different, but highly correlated,”
Ariel Sanchez points out. “To extract the most information about the
cosmological parameters, we had to find not only the best methods and
models for data analysis but also the optimal combination of these
measurements.”

This analysis has now born fruit: the BOSS data show that dark
energy, which is driving the cosmological expansion, is consistent with a
cosmological constant within an error of only 5%. This constant, called
Lambda, was introduced by Albert Einstein to counter the attractive
force of matter, i.e. it has a repellent effect. Moreover, all results
are fully consistent with the standard cosmological model, giving
further strength to this still relatively young theory.

In particular, the map also reveals the distinctive signature of the
coherent movement of galaxies toward regions of the Universe with more
matter, due to the attractive force of gravity. Crucially, the observed
amount of infall matches well to the predictions of general relativity.
This supports the idea that the acceleration of the expansion rate is
driven by a phenomenon at the largest cosmic scales, such as dark
energy, rather than a breakdown of our gravitational theory.